Dosimetric Impact of Pelvic Bone Electron Density Assignment In Online Adapt-to-Shape (ATS) MR-Guided Prostate SBRT
Abstract
Purpose
To quantify the dosimetric impact of pelvic bone electron density (ED) assignment on dose accuracy and target coverage in an adapt-to-shape (ATS) MR-guided adaptive radiotherapy (MRgRT) workflow on a 1.5 T MR-Linac for prostate stereotactic body radiotherapy (SBRT).
Methods
Ten treated prostate SBRT patients (40 Gy in 5 fractions) were retrospectively analyzed. Planning objectives and dose–volume constraints followed institutional directives based on Zelefsky et al. For each patient, a reference plan was generated by forcing pelvic bone ED to match the external/body ED, emulating the ATS scenario in which pelvic bone inherits the external/body ED when it is not contoured. Plans were normalized such that 95% of the planning target volume (PTV) received 40 Gy. Statistical uncertainty per calculation was set to 0.3%. Each reference plan was copied and recalculated as an evaluation plan using the actual pelvic bone ED (without the forced ED override) while keeping segment shapes and monitor units unchanged. Dosimetric endpoints included V40Gy, D95%, and D99% for the PTV and clinical target volume (CTV). Agreement between reference and evaluation plan dose distributions was assessed using 3D local gamma analysis with criteria of 2%/2 mm and a 10% dose threshold.
Results
Evaluation plans exhibited systematic loss of near-prescription coverage relative to reference. PTV V40Gy decreased from 95.00% ± 0.00% to 90.41% ± 2.25% (p < 0.05), and CTV V40Gy decreased from 98.69% ± 1.17% to 95.84% ± 2.13% (p < 0.05). Mean D95% and D99% for both PTV and CTV were reduced by 0.31–0.32 Gy. The mean 3D local gamma passing rate between reference and evaluation plans was 88.34% ± 3.43%.
Conclusion
In ATS MRgRT prostate SBRT, omitting pelvic bone contouring can overestimate target coverage. Contouring pelvic bone and assigning the appropriate ED improves dose calculation accuracy and target coverage evaluation during online adaptation.